Multilayer cholesteric pigments

ABSTRACT

A platelet-shaped cholesteric multilayer pigment which comprises the layer sequence A/B/ and if desired C, 
     where 
     A and C independently of one another are at least one partly light-permeable absorption layer, and 
     B is at least one cholesteric layer.

The invention relates to multilayer cholesteric pigments, to processesfor their preparation and to their use.

When substances exhibiting shape anisotropy are heated it is possiblefor liquid-crystalline phases known as mesophases to occur. Theindividual phases differ in the spatial arrangement of the centers ofmass of the molecules, on the one hand, and in the arrangement of themolecules with respect to the long axes, on the other hand. The nematicliquid-crystalline phase is distinguished by parallel orientation of thelong axes of the molecules (one-dimensional order state). Provided thatthe molecules forming the nematic phase are chiral, the result is achiral nematic (cholesteric) phase in which the long axes of themolecules form a helical superstructure perpendicular thereto. Thechiral moiety may be present in the liquid-crystalline molecule itselfor else may be added as a dopant to the nematic phase, inducing thechiral nematic phase. This phenomenon was first investigated oncholesterol derivatives.

The chiral nematic phase has special optical properties: a high opticalrotation and a pronounced circular dichroism resulting from selectivereflection of circularly polarized light within the chiral nematiclayer. The colors appear different depending on the angle of view anddepend on the pitch of the helical superstructure, which in turn dependson the twisting power of the chiral component. In this case, it ispossible, in particular by altering the concentration of a chiraldopant, to vary the pitch and thus the wavelength range of theselectively reflected light of a chiral nematic layer. Chiral nematicsystems of this type have interesting possibilities for practical use.

Cholesteric special-effect pigments and compositions comprising suchpigments are known.

EP-B-383 376 describes liquid-crystal pigments comprisingplatelet-shaped carrier particles some of which at least are coated withliquid-crystalline material. Coating takes place by dispersing theplatelet-shaped particles in a solvent in which liquid-crystallinematerial is dissolved, and then precipitating at least some of theliquid-crystalline material onto the particles. In the course of thisthe platelet-shaped carrier particles become fully or partly envelopedby the cholesteric. Uniform cholesteric layers arranged exactly parallelto the middle layer cannot be prepared by this process. The pigments areapparently not fully hiding, since they are said to be appliedpreferably to black surfaces.

DE-A-196 19 973 outlines, in a non-imitable manner, an idea for two- orthree-layer platelet-shaped interference pigments. The pigments areintended to have at least one layer which consists of liquid-crystallinepolymers whose mesogens are at least approximately in chiral-nematicand/or smectic and/or cholesteric order. Also provided in theinterference pigments is a light-absorbing layer which is absorbent forat least part of the visible spectrum of light. The pigments are to beobtainable by knife coating, rolling or spray application to a smoothsubstrate, curing of the thin film thus produced, application of thelight-absorbing layer, curing of this light-absorbing layer, optionalapplication and curing of a further film which coincides with the firstfilm in its composition and layer thickness, and removal and comminutionof the cured layer assembly. Specific pigments, however, are notdisclosed. As far as the material composition of the pigments isconcerned, all that is said is that “liquid-crystalline main-chain orside-chain polymers or mixtures thereof, liquid-crystalline oligomers oroligomer mixtures, or liquid-crystalline monomers or monomer mixtures,[come] into consideration” as liquid-crystalline polymers. There are noexamples regarding the preparation of the pigments or thepigment-containing coating formulations. The disclosure content ofDE-A-196 19 973 is therefore limited to purely theoretical discussionsof the idea of two- or three-layer pigments. Consequently, no technicalteaching is provided that is imitable by the skilled worker.

WO 94/22976 describes two-layer cholesteric pigments based on twodifferent polyorganosiloxanes from the company Wacker. The pigments areprepared in an extremely complex manner by separate coating of twopreviously nylon-coated glass plates with solutions of theabovementioned liquid crystals; rubbing of each liquid-crystal layer inorder to orient it; attachment of thermally deformable spacers to theglass plates; placing of the glass plates together with theircholesteric layers facing one another, and uniting of the cholestericlayers by thermal deformation of the spacers at elevated temperature ina vacuum, and also crosslinking of the united cholesteric layers. Thefilm thus obtainable is said, like the pigments obtainable from it bymilling, to have a thickness of approximately 10 μm. Despite the priorcoating of the glass plates with nylon, detachment of the film from theglass plates is apparently incomplete, so that residues of the film haveto be scratched off in order to obtain the pigments from the plates,which makes the preparation of the pigments even more complex. The ideaof three-layer pigments is merely outlined. These pigments cannot beprepared by the preparation process described for two-layer pigments. WO94/22976 therefore provides no technical teaching which is imitable bythe skilled worker and which would in any way provide three-layerpigments. The disclosure content is limited to purely theoreticaldiscussions of the structure of three-layer pigments.

The prior document DE-A-197 57 699 discloses plated-shaped cholestericmultilayer pigments having at least two cholesteric layers and at leastone interlayer separating these cholesteric layers from one another andabsorbing some or all of the light transmitted by the cholesteric layer.If this absorbing interlayer is made fully opaque, then given asufficient level of pigmentation the perceived color of the pigment isentirely independent of the background, thereby making such pigmentsvery highly suitable for use, for example, in automotive paints.

In order to absorb the transmitting wavelength range, other prior artcholesteric interference pigments must either contain additionalpigments in the cholesteric matrix or be applied to a coloredbackground. When foreign pigments are incorporated into theliquid-crystalline mass it is disadvantageous that a considerableportion of the reflecting wavelength range is absorbed or scattered byabsorption and scattered light, so that the special advantage of theinterference pigments on a cholesteric basis is largely removed. Thesame problem occurs if cholesteric pigments are mixed with absorbingpigments into coating formulations. Reflections which disrupt theperceived color can only be avoided if the absorbing pigment isdispersed very finely into the cholesteric matrix. From generalexperience this is only the case if the pigment is dispersed usingadditives tailored specifically to the pigment surface. These compounds,such as fatty acids, salts of fatty acids, soya lecithins or phosphates,however, interfere with the development of the helical orientation andthis prevents optimum color development. If, on the other hand,absorption takes place over a colored underlayer, the background must beof uniformly high quality in order to provide the desired overallimpression of the effect coating. Consequently, considerable effort hasto be expended on pretreating the background. An ideal background formaximum brilliance would have to be black or have specular gloss, whichin the case of car bodies, for example, would be extremely difficult torealize.

It is an object of the present invention to provide special-effectpigments which no longer have the above-described disadvantages of theprior art.

We have found that this object is achieved by a multilayer pigment whichcomprises at least one partly light-permeable layer, below that at leastone cholesteric layer and, if desired, below that, at least one furtherpartly light-permeable layer.

The present invention therefore provides a platelet-shaped cholestericmultilayer pigment which comprises the layer sequence A/B/ and ifdesired C,

where

A and C independently of one another are at least one partlylight-permeable absorption layer, and

B is at least one cholesteric layer.

The multilayer pigment of the invention offers a range of surprisingadvantages:

a) A and/or C can be applied very thinly, so that the pigments have amore favorable diameter-thickness ratio than LC pigments of the priorart.

b) The thin pigment platelets can be aligned more readily, especially inan automotive basecoat, than LC pigments of the prior art.

c) The pigments are more lustrous than the LC pigments of the prior art,so that in an automotive finish, for example, less clearcoat need beapplied to the basecoat film.

d) The opacity of the pigments is equal or superior to that of prior artpigments, since substantially more pigments particles of the inventioncan be employed per unit volume of paint.

e) The use of semitransparent pigments permits coating systems with adepth effect, which exhibit a pearlescent shimmer.

f) A and/or C can have the same or different coloration, which allowsfor numerous combinations of interference colors and absorption colors.For example, layers A and/or C absorbing in the blue wavelength rangecan cut out blue shades from an angle-dependent color play of the layerB.

A and/or C comprise preferably absorbing colorants. These can be presentin pigmentary form or in solution as a dye in an organic or inorganicbinder matrix. The absorption pigment can be an overcolored whitepigment, a color pigment or, preferably, a black pigment. Suitablepigments are nonabsorbing pigments (white pigments) which have beenmixed with absorbing colorants (dyes or colored pigments), examplesbeing TiO₂, ZrO₂, Al₂O₃, SiO₂, ZnO and SnO₂. Preference is given toemploying selectively or nonselectively absorbing pigments. Amongselectively absorbing pigments particular mention may be made of ironoxides, chromates, vanadates and sulfides; among those which arenonselectively absorbing, black Fe₃O₄ (magnetite) and carbon black.

Examples of suitable organic absorption pigments are azo pigments, metalcomplex pigments, such as azo- and azomethine-metal complexes,isoindolinone and isoindoline pigments, phthalocyanine pigments,quinacridone pigments, perinone and perylene pigments, anthraquinonepigments, diketopyrrolopyrrole pigments, thioindigo pigments, dioxazinepigments, triphenylmethane pigments, quinophthalone pigments andfluorescent pigments.

Particularly suitable absorption pigments are fine such pigments havingan average particle size of from 0.01 to 1 μm, preferably from 0.01 to0.1 μm.

It is preferably possible to employ various grades of carbon black,especially readily dispersible pigment-grade carbon blacks having aspecific surface area of from 30 to 150 m²/g (BET method) and anabsorption capacity of from 50 to 100 ml of dibutyl phthalate/100 g (DBPnumber).

Suitable absorption pigments also include those which impart magneticproperties to A and/or C. Suitable examples are γ-Fe₂O₃, Fe₃O₄, CrO₂ orferromagnetic metal pigments, such as Fe, Fe—Cu and Fe—Ni—Co alloys, forexample.

Absorption dyes which may be present in A and/or C are, in principle,always selectively or nonselectively absorbing dyes which are soluble inthe organic or inorganic binders employable in accordance with theinvention and which do not adversely affect the properties of the layerin which they are present. Examples of suitable dyes are monoazo dyesand their metal salts, disazo dyes, condensed disazo dyes, isoindolinederivatives, derivatives of naphthalene- or perylenetetracarboxylicacid, anthraquinone dyes, thioindigo derivatives, azomethinederivatives, quinacridones, dioxazines, pyrazoloquinazolones,phthalocyanine dyes or basic dyes such as triarylmethane dyes and saltsthereof.

The absorption pigments or dyes are preferably bound into, or dissolvedin, an organic binder matrix. Employable binders are the customarycoatings systems. Suitable systems are preferably radiation-curablesystems comprising reactive, crosslinkable groups, such as acrylic,methacrylic, α-chloroacrylic, vinyl, vinyl ether, epoxy, cyanate,isocyanate or isothiocyanate groups.

Other binders which can be employed are monomeric agents and mixturesthereof with polymeric binders. Preferred monomeric agents are thosewhich have two or more crosslinkable groups, such as acrylic,methacrylic, α-chloroacrylic, vinyl, vinyl ether, epoxy, cyanate,isocyanate or isothiocyanate groups. Particular preference is given toacrylic, methacrylic or vinyl ether groups. Examples of monomeric agentshaving two crosslinkable groups are the diacrylates, the divinyl ethersor the dimethacrylates of diols such as propanediol, butanediol,hexanediol, ethylene glycol, diethylene glycol, triethylene glycol ortetrapropylene glycol, for example.

Examples of monomeric agents having three crosslinkable groups are thetriacrylates, the trivinyl ethers or the trimethacrylates of triols suchas trimethylolpropane, ethoxylated trimethylolpropane having 1 to 20ethylene oxide units, propoxylated trimethylolpropane having 1 to 20propylene oxide units, and mixed ethoxylated and propoxylatedtrimethylolpropane in which the sum of ethylene oxide and propyleneoxide units is from 1 to 20. Examples of monomeric agents having threecrosslinkable groups are also the triacrylates the trivinyl ethers orthe trimethacrylates of glycerol, ethoxylated glycerol having 1 to 20ethylene oxide units, propoxylated glycerol having 1 to 20 propyleneoxide units, and mixed ethoxylated and propoxylated glycerol in whichthe sum of ethylene oxide and propylene oxide units is from 1 to 20.

Examples of monomeric agents having four crosslinkable groups are thetetraacrylates, the tetravinyl ethers or the tetramethacrylates oftetraols such as bis-trimethylolpropane, ethoxylatedbis-trimethylolpropane having 1 to 20 ethylene oxide units, propoxylatedbis-trimethylolpropane having 1 to 20 propylene oxide units, and mixedethoxylated and propoxylated bis-trimethylolpropane, in which the sum ofethylene oxide and propylene oxide units is from 1 to 20. Furtherexamples of monomeric agents having four crosslinkable groups are thetetraacrylates, the tetravinyl ethers or the tetramethacrylates oftetraols such as pentaerythritol, ethoxylated pentaerythritol having 1to 20 ethylene oxide units, propoxylated pentaerythritol having 1 to 20propylene oxide units, and mixed ethoxylated and propoxylatedpentaerythritol in which the sum of ethylene oxide and propylene oxideunits is from 1 to 20.

To enhance the reactivity in the course of crosslinking orpolymerization in air it is possible for the binders and the monomericagents to include from 0.1 to 10% of a primary or secondary amine.Examples of suitable amines are ethanolamine, diethanolamine ordibutylamine.

A suitable inorganic binder which may be present in A and/or C is, forexample, SiO₂, which can be applied in the form of waterglass orsilanes.

The absorption pigments and/or dyes are preferably stirred into a meltor solution of the inorganic binder or are admixed in another customarymanner. The pigmented or dye-containing binder is applied as describedlater on below.

With particular preference, the absorption pigments or dyes of A and/orC are bound into or dissolved in a binder matrix which comprises thecholesteric mixtures that are described further below as ingredients ofB. With very particular preference, the binder matrix of A and/or Ccomprises the same cholesteric mixtures as B.

The absorption pigment formulation can be prepared by the customarydispersion techniques, which are known in the art, and using diluents,dispersants, photoinitiators and, if desired, further additives.

The absorption dye of the formulation can be prepared in a known mannerby dissolving the dye in one of the above-mentioned binders or in abinder mixture with or without the use of diluents, photoinitiators andfurther additives.

Diluents which can be used are water or organic liquids or mixturesthereof, preference being given to organic liquids. Particularlypreferred organic liquids are those having a boiling point of below 140°C., especially ethers such as tetrahydrofuran, ketones such as ethylmethyl ketone and esters such as butyl acetate.

Dispersants which can be used are low molecular mass dispersants such asstearic acid, for example, or else polymeric dispersants. Suitablepolymeric dispersants or dispersing resins are known to the skilledworker. Mention may be made in particular of polyurethanes containingsulfonate, phosphonate or carboxyl groups, vinyl chloride copolymerscontaining carboxyl groups, polyimine polyesters or polyether acrylates,with or without functional groups incorporated. One group of preferreddispersants is known, for example, from DE-A-195 16 784 or from EP-A-0742 238, which are expressly incorporated herein by reference.

For the preparation of crosslinkable or polymerizable absorption pigmentformulations it is possible to use the photoinitiators customary forphotochemical polymerization, examples being the photoinitiators listedbelow for the photochemical polymerization of the cholesteric mixtures.

In another preferred embodiment of the present invention, A and/or Ccontain a preferably crosslinked polymer which shows selective ornonselective absorption for light. Examples of nonselectively absorbingpolymers are black polymers, such as polypyrrole. Examples ofselectively absorbing polymers are colored polymers. The absorbingpolymers can comprise the above-mentioned absorption pigments orabsorption dyes in dispersed form or in dissolved form, respectively, inorder to vary or intensify the desired absorption effect. The absorbingpolymers can also include chromophores attached covalently to thepolymer.

Another preferred embodiment of the present invention is a multilayerpigment where A and/or C comprise at least one essentially inorganic,light-absorbing absorption layer. For the purposes of the presentinvention a layer is defined as essentially inorganic if it comprisesorganic compounds (carbon compounds) merely as impurities (≦5% byweight). The inorganic absorption layer can comprise both selectively ornonselectively absorbing substances of high refractive index or elsenonselectively absorbing substances which are of low refractive indexbut have a high absorption constant and which, of course, must also beable to be deposited in a permanent, filmlike manner. Such substancesare described, for example, in EP-A-753 545, the full content of whichis incorporated herein by reference.

Examples of materials of high refractive index suitable for A and/or Care nonselectively absorbing materials, such as metals, metal oxides,metal sulfides and mixtures thereof, which may also comprise selectivelyabsorbing metal oxides in minor amounts, and selectively absorbingmaterials, such as metal oxides in particular, which generally have ineach case a refractive index n>2.0, preferably n>2.4.

Specifically, the following materials may be mentioned as examples ofnonselectively absorbing materials of high refractive index that aresuitable for A and/or C:

materials which can be applied by gas-phase decomposition of volatilemetal compounds, such as molybdenum, iron, tungsten, chromium, cobaltand nickel, and mixtures of these metals; metals which can be depositedby wet-chemical means, through reduction from metal salt solutions, suchas silver, copper, gold, palladium and platinum, and also cobalt andnickel and alloys such as NiP, NiB, NiCo, NiWP, CoP and AgAu;

metal oxides, such as magnetite Fe₃O₄, cobalt oxide (CoO, Co₃O₄) andvanadium oxide (VO₂, V₂O₃), and also mixtures of these oxides with themetals, such as magnetite and iron in particular;

metal sulfides, such as molybdenum sulfide, iron sulfide, tungstensulfide, chromium sulfide, cobalt sulfide and nickel sulfide, andmixtures of these sulfides such as MoS₂/WS₂, and also, in particular,mixtures of these sulfides with the respective metal, such as inparticular MoS₂ and molybdenum, and mixtures with oxides of therespective metal, such as MoS₂ and molybdenum oxides.

Also suitable as nonselectively absorbing absorption layers of highrefractive index, for example, are layers of colorless materials of highrefractive index, such as zirconium dioxide and, in particular, titaniumdioxide, into which nonselectively absorbing (black) material (e.g.carbon) is incorporated, or which are coated with said material.

Examples of selectively absorbing materials of high refractive index areespecially colored oxides such as preferably iron(II) oxide (α- andγ-Fe₂O₃, red), chromium(III) oxide (green), titanium(III) oxide (Ti₂O₃,blue) and vanadium pentoxide (orange), and also colored nitrides, suchas preferably titanium oxynitride and titanium nitride (TiO_(x)N_(y),TiN, blue), the lower titanium dioxides and titanium nitrides generallybeing present as a mixture with titanium dioxide.

It is of course also possible here to use colorless materials of highrefractive index, examples being metal oxides such as zirconium oxideand especially titanium oxide, which are colored with selectivelyabsorbing colorants, something which may be effected by incorporatingcolorants into the metal oxide layer, by doping thereof with selectivelyabsorbing metal cations, or by overcoating the metal oxide layer with afilm comprising a colorant.

Finally, suitable nonselectively absorbing materials of low refractiveindex with high absorption constants for A and/or C are, in particular,metals such as aluminum.

The coating should not be opaque but should be at least partly permeableto visible light (semitransparent) and is, therefore, of varyingthickness depending on the optical properties of the selected layermaterials.

The thickness of the individual layers of A and/or C when usinginorganic absorption layers for nonselectively absorbing materials ofhigh refractive index, such as metals, black metal oxides and sulfides,is generally from 1 to 100 nm, with preference being given, for stronglyabsorbing metals, such as molybdenum and chromium, to layer thicknessesof from about 1 to 25 nm, for less strongly absorbing materials, such asmagnetite, to layer thicknesses of from about 10 to 50 nm, and, formetal sulfide-containing materials, such as layers containing MoS₂, tolayer thicknesses of from 5 to 20 nm.

When using absorption pigments or absorbing polymers, the thickness ofeach individual layer of A and/or C is from about 1 nm to 5 μm, inparticular from about 5 nm to 3 μm, for example from about 5 to 700 nmor from 5 to 500 nm or from 5 to 300 nm.

A and C may also be identical or different in terms of their mechanicalproperties. They may, for example, differ in thickness or brittleness.

Preferably, the thickness of each individual cholesteric layer of B isfrom about 0.5 to 20 μm, in particular from about 1 to 10 μm and, withparticular preference, from about 2 to 4 μm or from 1 to 2.5 μm or from1 to 1.5 μm.

The diameter of the pigments of the invention is from about 3 to 500 μm,in particular from about 3 to 100 μm or from 10 to 100 μm and, withparticular preference, from about 3 to 30 μm. In general, the pigmentdiameter is approximately from 5 to 20 times the pigment thickness.

The total thickness of the multilayer pigments of the invention is, forexample, less than about 30 μm, or less than about 20 μm or less thanabout 10 μm, such as, for example, from about 1 to 5 μm or from 1 to 3μm.

Advantageously, the adhesion between cholesteric layer B and absorptionlayers A and/or C is chosen so that there is essentially no delaminationwhen the layer assembly is ground to the pigment. The adhesive forcebetween cholesteric layer and absorption layer is judiciously more thanabout 8 cN, in particular more than about 12 cN, such as, for example,from about 15 to 30 cN or from 18 to 25 cN. The determination of theadhesive force is described in more detail in the experimental section.

Suitable in accordance with the invention are also pigments whoseabsorption layer(s) A and/or C are magnetic. Such pigments can,advantageously, be given an arbitrary orientation by application of amagnetic field. In this way it is possible, for instance, to preventindividual pigment platelets projecting from the others, which resultsin the scattering of less light and an improvement in the perceivedcolor. All of the platelets can be oriented together in a defined angle.It is also possible to generate full-area patterns in order to obtainnew color effects, or partial patterns for optical emphasis of indiciaor structures. The magnetic cholesteric pigments of the invention canalso be employed with advantage in a liquid matrix, for example in LCDs,where they alter their alignment and therefore their perceived colorwhen a magnetic field is applied.

The layer assembly B of the pigments of the invention preferablycomprises cholesteric mixtures selected from

a) at least one cholesteric, polymerizable monomer;

b) at least one achiral, nematic, polymerizable monomer and one chiralcompound;

c) at least one cholesteric, crosslinkable polymer; or

d) a cholesteric polymer in a polymerizable diluent;

e) at least one cholesteric polymer whose cholesteric phase can befrozen in by rapid cooling to below the glass transition temperature,

in the cured state.

Curing fixes the uniform orientation of the cholesteric molecules in thecholesteric layer.

Preferred monomers of group a) are described in DE-A-196 02 848, thefull content of which is incorporated herein by reference. Inparticular, the monomers a) embrace at least one chiral,liquid-crystalline, polymerizable monomer of the formula I

[Z¹—Y¹—A¹—Y²—M¹—Y³]_(n)X  (I)

where

Z¹ is a polymerizable group or a radical which carries a polymerizablegroup,

Y¹, Y², Y³ independently are chemical bonds, oxygen, sulfur, —CO—O—,—O—CO—, —O—CO—O—, —CO—N(R)— or —N(R)—CO—,

A¹ is a spacer,

M¹ is a mesogenic group,

X is an n-valent chiral radical,

R is hydrogen or C₁-C₄-alkyl,

n is 1 to 6,

and Z¹, Y¹, Y², Y³, A¹ and M¹ can be identical or different if n isgreater than 1.

As preferred monomers of group b), the cholesteric mixture in theprocess of the invention includes at least one achiralliquid-crystalline polymerizable monomer of the formula II

Z²—Y⁴—A²—Y⁵—M²—Y⁶—A³—(Y⁷—Z³)_(n)  (II)

where

Z², Z³ are identical or different polymerizable groups or radicals whichcontain a polymerizable group,

n is 0 or 1,

Y⁴, Y⁵, Y⁶, Y⁷ independently are chemical bonds, oxygen, sulfur, —CO—O—,—O—CO—, —O—CO—O—, —CO—N(R)— or —N(R)—CO—,

A², A³ are identical or different spacers and

M² is a mesogenic group.

In addition, the mixture of group b) includes a chiral compound. Thechiral compound brings about the twisting of the achiralliquid-crystalline phase to form a cholesteric phase. In this context,the extent of twisting depends on the twisting power of the chiraldopant and on its concentration. Consequently, therefore, the pitch ofthe helix and, in turn, the interference color depend on theconcentration of the chiral dopant. As a result, it is not possible toindicate a generally valid concentration range for the dopant. Thedopant is added in the amount at which the desired color effect isproduced.

Preferred chiral compounds combinable with the achiral compounds of theformula II are those of the formula Ia

[Z¹—Y¹—A¹—Y²—M^(a)—Y³—]_(n)X  (Ia),

where Z¹, Y¹, Y², Y³, A¹, X and n are as defined above and M^(a) is adivalent radical which comprises at least one heterocyclic or isocyclicring system.

The moiety M^(a) here is similar to the mesogenic groups described,since in this way particularly good compatibility with theliquid-crystalline compound is achieved. M^(a), however, need not bemesogenic, since the compound Ia is intended merely by means of itschiral structure to bring about the appropriate twisting of theliquid-crystalline phase. Further monomers and chiral compounds of groupb) are described in WO 97/00600 and its parent DE-A-195 324 08, the fullcontent of which is expressly incorporated herein by reference.

Preferred polymers of group c) are cholesteric cellulose derivatives asdescribed in DE-A-197 136 38, especially cholesteric mixed esters of

(VI) hydroxyalkyl ethers of cellulose with

(VII) saturated, aliphatic or aromatic carboxylic acids and

(VIII) unsaturated mono- or dicarboxylic acids.

It is also possible to use crosslinkable oligosiloxanes or polysiloxanesas are described, for example, in EP-A-0 358 208, DE-A-195 41 820 andDE-A-196 19 460.

Highly suitable polymers of group c), moreover, are thepropargyl-terminated cholesteric polyesters or polycarbonates describedin DE-A-197 17 371.

Further suitable polymers of group c) are cholesteric polycarbonatescontaining photoreactive groups even in a nonterminal position. Suchpolycarbonates are described in DE-A-196 31 658.

Suitable polymers of group e) are chiral nematic polyesters havingflexible chains and comprising isosorbide, isomannide and/or isoidideunits, preferably isosorbide units, and also comprising at least onechain-flexibilizing unit selected from (and derived from)

(a) aliphatic dicarboxylic acids,

(b) aromatic dicarboxylic acids with a flexible spacer,

(c) α,ω-alkanoids,

(d) diphenols with a flexible spacer, and

(e) condensation products of a polyalkylene terephthalate orpolyalkylene naphthalate with an acylated diphenol and with an acylatedisosorbide,

as are described in DE-A-197 04 506.

The polyesters are noncrystalline and form stable Grandjean textureswhich can be frozen in on cooling to below the glass transitiontemperature. The glass transition temperatures of the polyesters are inturn, despite the flexibilization, above 80° C., preferably above 90° C.and, in particular, above 100° C.

Examples of preferred polymers of group d) are crosslinkable cholestericcopolyisocyanates as described in U.S. Pat. No. 08,834,745.

Very particular preference is given, in accordance with the invention,to the presence in layer B of chiral compounds and nematic monomers ofgroup b), especially of chiral compounds of the formula 2:

or of the formula 5:

and nematic monomers of the formula 1:

or preferably of the formula 3:

or with particular preference of the formula 4:

in the cured state, where n₁, and n₂ in formulae 1 and 3 areindependently 4 or 6, R′ in formula I is H or Cl, and the monomers ofthe formula 1 or 3 are preferably employed as mixtures of the compoundswith n₁/n₂=4/4, 4/6, 6/4 or 6/6, and R in formula 4 is H, Cl or CH₃. Itis also possible in accordance with the invention, however, for othercholesteric mixtures, examples being the mixtures disclosed in EP-A-686674, to be present in the cured state in B.

The cholesteric mixtures, and the formulations comprising absorptionpigment, can be diluted with any suitable diluent before being appliedto the carrier.

Diluents which can be employed in the process of the invention for thecompounds of the groups a) and b) are linear or branched esters,especially acetic esters, cyclic ethers and esters, alcohols, lactones,aliphatic and aromatic hydrocarbons, such as toluene, xylene andcyclohexane, and also ketones, amides, N-alkylpyrrolidones, especiallyN-methylpyrrolidone, and in particular tetrahydrofuran (THF), dioxaneand methyl ethyl ketone (MEK).

Examples of suitable diluents for the polymers of group c) are ethersand cyclic ethers, such as tetrahydrofuran or dioxane, chlorinatedhydrocarbons, such as dichloromethane, 1,1,2,2-tetrachloroethane,1-chloronaphthalene, chlorobenzene or 1,2-dichlorobenzene. Thesediluents are particularly suitable for polyesters and polycarbonates.Examples of suitable diluents for cellulose derivatives are ethers, suchas dioxane, or ketones, such as acetone. If copolyisocyanates areemployed as polymers of group d) it is advisable to use polymerizablediluents as described in U.S. Pat. No. 8,834,745.

The mixtures of groups a), b) or c) may also include, in small amounts,polymerizable diluents in addition to the inert diluent. Preferredpolymerizable solvents which can be added to a), b) or c) are acrylates,especially acrylates of relatively high functionality such as bis-,tris- or tetraacrylates, and with particular preference high-boilingoligoacrylates. The preferred amount added is approximately 5% byweight, based on the overall weight of the mixture.

For photochemical polymerization, the cholesteric mixture may includecustomary commercial photoinitiators. For curing by electron beams, suchinitiators are not required. Examples of suitable photoinitiators areisobutyl benzoin ether, 2,4,6-trimethylbenzoyldiphenylphosphine oxide,1-hydroxycyclohexyl phenyl ketone,2-benzyl-2-dimethylamino-l-(4-morpholinophenyl)furan-1-one, mixtures ofbenzophenone and 1-hydroxycyclohexyl phenyl ketone,2,2-dimethoxy-2-phenylacetophenone, perfluorinated diphenyltitanocenes,2-methyl-1-(4-[methylthio]-phenyl)-2-(4-morpholinyl)-1-propanone,2-hydroxy-2-methyl-1-phenylpropan-l-one, 4-(2-hydroxyethoxy)phenyl2-hydroxy-2-propyl ketone, 2,2-diethoxyacetophenone,4-benzoyl-4′-methyldiphenyl sulfide, ethyl 4-(dimethylamino)benzoate,mixtures of 2-isopropylthioxanthone and 4-isopropylthioxanthone,2-(dimethylamino)ethyl benzoate, d,l-camphorquinone,ethyl-d,l-camphorquinone, mixtures of benzophenone and4-methylbenzophenone, benzophenone, 4,4′-bisdimethylaminobenzophenone,(η⁵-cyclopentadienyl)(η⁶-isopropylphenyl)iron(II) hexafluorophosphate,triphenylsulfonium hexafluorophosphate or mixtures of triphenylsulfoniumsalts, and also butanediol diacrylate, dipropylene glycol diacrylate,hexanediol diacrylate, 4-(1,1-dimethylethyl)cyclohexyl acrylate,trimethylolpropane triacrylate and tripropylene glycol diacrylate.

The brightness of the cholesteric layer(s) B can be increased by addingsmall amounts of suitable leveling agents. It is possible to employ fromabout 0.005 to 1% by weight, in particular from 0.01 to 0.5% by weight,based on the amount of cholesteric employed. Examples of suitableleveling agents are glycols, silicone oils and, in particular, acrylatepolymers, such as the acrylate copolymers obtainable under the name Byk361 or Byk 358 from Byk-Chemie and the modified, silicone-free acrylatepolymers obtainable under the name Tego flow ZFS 460 from Tego.

Preferably, A and/or C and if desired B also include, in amounts fromabout 0.1 to 10% by weight, stabilizers to counter the effects of UV andweather. Examples of suitable such additives are derivatives of2,4-dihydroxybenzo-phenone, derivatives of 2-cyano-3,3-diphenylacrylate, derivatives of 2,2′,4,4′-tetrahydroxybenzophenone, derivativesof ortho-hydroxyphenylbenzotriazole, salicylic esters,orthohydroxyphenyl-S-triazines or sterically hindered amines. Thesesubstances can be employed alone or, preferably, as mixtures.

The present invention additionally provides a process for producing apigment of the invention, which comprises applying the layers A, B andif desired C atop one another to a substrate, simultaneously or with atime differential, curing the layers by heat, UV radiation, electronbeams or by rapid cooling to below the glass transition temperature,again simultaneously or with a time differential, removing the layerstogether from the substrate, and then comminuting them to give pigments.

The application of the layers A, B and if desired C to the substrate canbe carried out by means of customary techniques selected, for example,from air-knife coating, knife coating, air blade coating, squeezecoating, impregnation, reverse roll coating, transfer roll coating,gravure coating, kiss coating, casting, spray coating, spin coating orprinting techniques, such as letterpress (relief), intaglio,flexographic, offset or screen printing. The layers A, B and if desiredC are preferably applied to the substrate by means of casting or offsetprinting.

If A and/or C comprise an essentially inorganic absorption layer, it canbe applied by means of physical vapor deposition (PVD) or, inparticular, by means of chemical vapor deposition (CVD) or by means ofwet-chemical precipitation methods. Where A and/or C include not onlythe inorganic absorption layer but also further layers—for example,layers comprising colored polymer or pigmented or dye-containingabsorption layers—the inorganic absorption layer can be applied prior toor following the application of these other layers.

The present invention additionally provides a platelet-shapedcholesteric multilayer pigment which comprises at least one cholestericlayer B enveloped by at least one partly light-permeable absorptionlayer A, with A being essentially inorganic and B being composed asdescribed above. The present invention additionally provides a processfor preparing such a multilayered pigment, which comprises firstapplying layer B to a substrate, curing it, removing the cured layerfrom the substrate, comminuting it to form pigments, and then applyinglayer A to the pigments.

The application of the essentially inorganic layers A and/or C can becarried out, for example, by wet chemical means or by means of CVD, withthe coating conditions being chosen so as not to damage B.

Metallic layers A and/or C are preferably applied by decomposition ofmetal carbonyls such as iron pentacarbonyl, chromium, molybdenum ortungsten hexacarbonyl, nickel tetracarbonyl and/or dicobalt octacarbonylat from 70 to 300° C. under inert conditions. For the decomposition ofMo(CO)₆, which is particularly preferred, suitable temperatures in thiscase are in particular from 200 to 250° C.

For conducting the CVD variant it is advisable, as generally for CVDtechniques, to use a fluidized-bed reactor. The pigments, composed ofone layer or a plurality of layers B, are heated in the reactor to thedesired reaction temperature (generally from 100 to 300° C., preferablyfrom 150 to 200° C.) with fluidization using an inert fluidizing gas,such as nitrogen.

Aluminum layers A and/or C can be deposited by means of inert gas-phasedecomposition of aluminum organyls, especially aluminum alkyls oralkylamine adducts of aluminum hydrides.

Suitable aluminum alkyls are not only monoalkylaluminum hydrides andhalides but also, preferably, dialkylaluminum hydrides and halides, and,in particular, aluminum trialkyls, such as triethyl- andtrimethylaluminum in particular.

In terms of the process, the procedure when applying the aluminum layersA and/or C is judiciously such that the aluminum alkyl as a solution ina relatively nonvolatile hydrocarbon, such as petroleum, is charged toan evaporator vessel, which is located upstream of the coating reactorand is heated in stages to about 100-150° C., then transferred into thereactor with the aid of a stream of inert gas (e.g., argon or, inparticular, nitrogen) which is passed through this solution, transfertaking place by way of a preferably thermally conditioned nozzle, and inthe reactor is subjected to thermal decomposition at generally from 100to 300° C., preferably from 150 to 400° C.; in general, the amount ofgas of the volatile aluminum compound should not be more than 2% byvolume, preferably not more than 1% by volume, of the overall amount inthe reactor.

As the reactor, particular preference is given to the above-mentionedfluidized-bed reactor, although it is also possible to use asingle-necked round-bottomed flask made of quartz glass which is rotatedby means of a motor, is provided with incoming and outgoing gas lines inthe axis of rotation, and is heated by a double-shelled valve oven(rotary sphere oven). In principle it is also possible as the reactor toemploy any heatable mixer which by means of appropriate internals gentlymoves the pigments composed of B and allows gas to enter and leave. Alsosuitable for a continuous process regime on the industrial scale, forexample, is a rotary tube furnace to which the pigments composed of Band the aluminum alkyl/inert gas mixture are supplied continuously.

Metallic layers A and/or C can, finally, also be applied by wet chemicalmeans through reduction of suitable metal salt solutions. In this way itis possible in particular to deposit relatively noble metals, such asespecially silver, copper, gold, cobalt, nickel, palladium and platinum.A series of reducing agents are suitable for this purpose, especiallymild organic reducing agents, examples being sugars, such as glucose anddextrose, and also formaldehyde.

In general, however, the metal layers applied by way of the gas phaseare preferred over those applied by wet chemical means, owing to theirhigher quality (more finely crystalline, film-like), since theygenerally result in brighter pigments of higher color strength.

For the chemical vapor deposition (CVD) of nonselectively absorbinglayers B, which consist essentially of lower metal oxides (e.g., Fe₃O₄,VO₂, V₂O₃), preference is given to decomposing metal carbonyls, such asiron pentacarbonyl, or oxychlorides, such as vanadium oxychloride, withsteam. If the gas-phase decomposition gives rise first to higher metaloxides, such as V₂O₅, these must be subsequently reduced to the desiredoxide using, for example, hydrogen or ammonia.

The CVD techniques already described are also particularly suitable forgenerating selectively absorbing layers B, which consist essentially ofcolored metal oxides and/or metal nitrides.

The deposition of α-iron(III) oxide, chromium(III) oxide andtitanium(III) oxide by oxidative decomposition of iron pentacarbonyl andchromium hexacarbonyl is sufficiently well known from the prior art.

Wet chemically, α-Fe₂O₃ and Cr₂O₃ layers can be applied by hydrolyticdecomposition of iron(III) salts, such as iron(III) chloride andiron(III) sulfate, or chromium(III) chloride, and subsequent conversionof the hydroxide-containing layers formed into the oxide layers by heattreatment.

Coating with selectively absorbing γ-Fe₂O₃ (B) can be carried out byknown CVD process variants involving first decomposing Fe(CO)₅ in thepresence of steam to deposit a magnetite film which is subsequentlyoxidized with air to give γ-Fe₂O₃, or by first oxidatively decomposingFe(CO)₅ to deposit an α-Fe₂O₃ film which is reduced with hydrogen toform products containing iron(III) and is then oxidized with air to giveγ-Fe₂O₃.

Vanadium(V) oxide layers B, finally, can be deposited by gas-phasedecomposition of vanadium oxychloride with steam.

To prepare colored TiO₂ layers, reference may be made to the details inDE-A-44 37 753.

The substrate to be coated in accordance with the invention ispreferably mobile and with particular preference is a moving substratein strip form.

Suitable layer substrates are, preferably, known films formed frompolyesters, such as polyethylene terephthalate or polyethylenenaphthalate, and also polyolefins, cellulose triacetate, polycarbonates,polyamides, polyimides, polyamidoimides, polysulfones, aramids oraromatic polyamides. The thickness of the layer substrates is preferablyfrom about 5 to 100 μm, in particular from about 10 to 20 μm. The layersubstrate can be subjected beforehand to a corona discharge treatment, aplasma treatment, a gentle adhesion treatment, a heat treatment, adedusting treatment or the like. The layer substrate preferably has amean center-line surface roughness of 0.03 μm or less, in particular of0.02 μm or less, and, with particular preference, 0.01 μm or less. It isdesirable, moreover, for the substrate to have not only a low meancenter-line surface roughness of this kind but also to possess no greatprojections (raised areas) of 1 μm or more. The roughness profile of thesurface of the substrate can be varied by means of fillers which areadded to the layer substrate in the course of its production. Examplesof suitable fillers are oxides and carbonates of Ca, Si and Ti, and fineorganic powders of acrylic substances.

The substrate can also be a metallized foil, a preferably polished metalstrip, or a cylinder roll.

B and layers A and C containing organic compounds can be of low or highviscosity, but are preferably of low viscosity, when they are applied tothe substrate. For this purpose, the cholesteric mixtures, or theformulations comprising absorption pigment, can be applied to thecarrier in undiluted or minimally diluted form at elevated temperatureor in highly diluted form at a low temperature. It is particularlypreferred to apply the three layers A, B and if desired C wet-on-wet tothe carrier in one operation, then to dry them together, if appropriate,and subsequently to subject them to conjoint curing.

For the simultaneous wet-on-wet application of said layers it isparticularly preferred if A and/or C comprise absorption pigments ordyes bound in a matrix of the same cholesteric mixture which is alsopresent in B. By this means, possibly disruptive layer boundariesbetween A, B and C are avoided, giving a homogeneous system comprisingpigments distributed homogeneously, or dyes dissolved uniformly, in thecentral region.

Casting techniques are particularly suitable for the simultaneousapplication of said layers, especially knife or bar coating processes,cast-film extrusion or stripping processes, and the cascade coatingprocess.

In the case of the knife or bar coating process, the liquid is appliedto a substrate through a slot in a casting block, the layer thicknessbeing adjustable by way of a defined knife or bar gap between a roller,over which the carrier is guided, and the lip of the coater. To applythe bottom (first) layer, the first casting block is brought toward theroller; to apply the second layer, a second casting block is broughttoward the first casting block and, to apply the third layer, a thirdcasting block is brought against the second. Both or all three liquidsrun to their respective coating blade or bar and are coated outsimultaneously over one another.

In the case of the cast-film extrusion or stripping process, a flexiblesubstrate, such as a film, is guided past the coater head under definedweb tension between two rollers. The amounts of liquid appropriate tothe desired layer thickness are applied simultaneously to the substratefrom three parallel casting slots arranged transverse to the runningdirection of the web.

In the cascade coating process, the substrate is guided over a roller.The liquids to be applied run over one another from differently arrangedslots and then run together onto the substrate.

It is of course also possible first to apply only one cholesteric layer,to subject this layer, if desired, to drying and to curing, and then toapply two layers wet-on-wet to the cured cholesteric layer by means, forexample, of one of the abovementioned processes. It is likewise possibleto subject each layer to individual and successive application, optionaldrying and curing.

If casting techniques are employed, the pourable cholesteric mixturepreferably has a viscosity in the range from about 10 to 500 mPas, inparticular from about 10 to 100 mPas, measured at 23° C. The cholestericmixture is, with particular preference, applied to the substrate at arate from about 1 to 800 m/min, in particular from about 5 to 100 m/min.It is preferred to use a casting apparatus whose casting slot width isin the range from about 2 to 50 μm, in particular from about 4 to 15 μm.The cholesteric mixture is preferably applied under elevated pressure,in particular at a coater overpressure in the range from about 0.01 to0.7 bar, with particular preference from 0.05 to 0.3 bar.

The cured layers can be removed from the substrate, for example, byguiding the substrate over a deflecting roller having a small diameter.As a consequence of this the crosslinked material then peels away fromthe substrate. Other known methods are equally suitable: for example,the stripping of the substrate over a sharp edge, or by way of an airknife, ultrasound or combinations thereof. The cholesteric material, nowdevoid of its substrate, is comminuted to a desired particle size. Thiscan be done, for example, by grinding in universal mills. In order tonarrow the particle size distribution the comminuted pigments cansubsequently be classified by means, for example, of a sieving process.

The invention additionally provides compositions comprising pigments ofthe invention.

Particularly preferred compositions of the invention are coatingmaterials, such as paints and varnishes, which comprise not only thepigments of the invention but also one or more substances selected fromwaterborne coatings, for example in the form of aqueous dispersions,such as PMA, SA, polyvinyl derivatives, PVC, polyvinylidene chloride, SBcopolymer, PV-AC copolymer resins, or in the form of water-solublebinders, such as shellac, maleic resins, rosin-modified phenolic resins,linear and branched, saturated polyesters, amino resin-crosslinkingsaturated polyesters, fatty acid-modified alkyd resins, plasticized urearesins, or in the form of water-thinnable binders, such as PUdispersions, EP resins, urea resins, melamine resins, phenolic resins,alkyd resins, alkyd resin emulsions, silicone resin emulsions; powdercoatings, such as powder coatings for TRIBO/ES, such as polyestercoating powder resins, PU coating powder resins, EP coating powderresins, EP/SP hybrid coating powder resins, PMA coating powder resins,or powder coatings for fluidized-bed sintering, such asthermoplasticized EPS, LD-PE, LLD-PE, HD-PE; solventborne coatings, suchas one- and two-component coating materials (binders) examples beingshellac, rosin esters, maleate resins, nitrocelluloses, rosin-modifiedphenolic resins, physically drying saturated polyesters, aminoresin-crosslinking saturated polyesters, isocyanate-crosslinkingsaturated polyesters, self-crosslinking saturated polyesters, alkydswith saturated fatty acids, linseed oil alkyd resins, soya oil resins,sunflower oil alkyd resins, safflower oil alkyd resins, ricinene alkydresins, tung oil/linseed oil alkyd resins, mixed-oil alkyd resins,resin-modified alkyd resins, styrene/vinyltoluene-modified alkyd resins,acrylicized alkyd resins, urethane-modified alkyd resins,silicone-modified alkyd resins, epoxy-modified alkyd resins, isophthalicacid alkyd resins, unplasticized urea resins, plasticized urea resins,melamine resins, polyvinyl acetals, noncrosslinking P(M)A homo- orcopolymers, noncrosslinking P(M)A homo- or copolymers with nonacrylicmonomers, self-crosslinking P(M)A homo- or copolymers, P(M)A copolymerswith other nonacrylic monomers, externally crosslinking P(M)A homo- orcopolymers, externally crosslinking P(M)A copolymers with nonacrylicmonomers, acrylate copolymer resins, unsaturated hydrocarbon resins,organic-soluble cellulose compounds, silicone combination resins, PUresins, P resins, peroxide-curing unsaturated synthetic resins,radiation-curing synthetic resins, both photoinitiator-containing andphotoinitiator-free radiation-curing synthetic resins; solvent-freecoating materials (binders) such as isocyanate-crosslinking saturatedpolyesters, two-pack PU resin systems, moisture-curing 1-component PUresin systems, EP resins, and also synthetic resins—individually or incombination—such as acrylonitrile-butadiene-styrene-copolymers, BS,cellulose acetate, cellulose acetobutyrate, cellulose acetopropionatecellulose nitrate, cellulose propionate, artificial horn, epoxy resins,polyamide, polycarbonate, polyethylene, polybutylene terephthalate,polyethylene terephthalate, polymethyl methacrylate, polypropylene,polystyrene, polytetrafluoroethylene, polyvinyl chloride, polyvinylidenechloride, polyurethane, styrene-acrylonitrile copolymers, or unsaturatedpolyester resins in the form of granules, powders or casting resin.

The compositions of the invention may additionally comprise stabilizersto counter the effects of UV and weather, and also inorganic or organicpigments, as described above.

The pigments of the invention can be incorporated individually or inmixtures into the compositions of the invention where they may ifdesired be subjected to additional alignment by methods which initiateshear forces. Suitable methods of aligning the pigments of the inventionare printing and knife coating or, in the case of magnetic pigments,applying an external magnetic field.

The present invention additionally provides coating materials comprisingat least one multilayer pigment of the invention, preferably coatingmaterials selected from effect paints, effect inks or effect films, andespecially from self-opacifying effect paints, inks or films.

The present invention also provides for the use of the pigments of theinvention for coloring plastics or films in the vehicle and vehicleaccessories sector, in the leisure, sport and games sector, indecorative cosmetics, in the textile, leather or jewelry field, in thegift product field, in writing utensils, packaging or spectacle frames,in the construction sector, in the household sector and in connectionwith printed products of all kinds, such as cardboard packaging, otherpackaging materials, carrier bags, papers or labels.

The color effects which can be achieved by means of the cholestericpigments of the invention embrace—owing to the host of achievablereflection wavelengths—the UV and IR region as well as, of course, theregion of visible light. If the pigments of the invention are applied toor incorporated into bank notes, check cards, other cashless means ofpayment or ID (by means, for example, of known printing techniques),this considerably hinders the identical copying, and especially thecounterfeiting, of these articles. The present invention thereforeadditionally provides for the use of the pigments of the invention forthe anticounterfeiting treatment of articles, especially bank notes,check cards or other cashless means of payment or ID.

Also provided for by the present invention is the use of thecompositions of the invention for coating articles of utility and forpainting vehicles.

The invention will now be elucidated further on the basis of thefollowing practical examples and with reference to the attached figure,wherein

FIG. 1: shows the diagrammatic representation of a coating apparatuswhich may be used in accordance with the invention.

EXAMPLE Preparation of a Semitransparent Cholesteric Effect Pigment

a) Preparing the Cholesteric Layer

The cholesteric layer was prepared by the casting method described in DE197 38 369.6 or PCT/EP 98/05544, which are expressly incorporated hereinby reference.

Use was made of a cholesteric mixture of the above-described group b)comprising as chiral monomer a compound of the formula 2 indicated aboveand as achiral, nematic monomer a mixture of compounds of the formula 3indicated above. The undiluted cholesteric mixture contained 90.5% byweight of the achiral, nematic compound, 6.5% by weight of the chiralcompound and, as photoinitiator, 3% by weight of 1-hydroxycyclohexylphenyl ketone, which is marketed under the designation Irgacure 184. Thecholesteric mixture was diluted with methyl ethyl ketone to a solidscontent of 50% by weight, based on the overall weight of the coatingcomposition. 0.1% by weight of Byk 361 was added as leveling agent.

Coating was carried out with a coating apparatus which is showndiagrammatically in FIG. 1. A polyethylene terephthalate film (PET film)(G) having a thickness of 15 μm was unrolled continuously from the filmwinder (F) and coated using a knife coater. The coating overpressure wasapproximately 0.2 bar. Drying took place in the dryer (C) at 60° C. Theresidence time in the dryer was 12 seconds. The layer was cured by UVfixing in the UV unit (A), while the dried strip was passed over thecooling roll (B). The cured cholesteric layer was wound up on the roller(D). The thickness of the dry cholesteric layer was 2.1 μm.

b) Applying the Absorbing Layer

For application to the cholesteric layer obtained in accordance withstage a), a coating composition of the following makeup was prepared:

In a laboratory kneading apparatus having a useful volume of 300 ml, 150g of pigment-grade carbon black Regal 400R (Cabot Corporation) werekneaded with 3 g of stearic acid, 80 g of a phosphorus dispersing resin(Disperdur phosphonate; K value=22; M_(w)=11,000) at 50% intetrahydrofuran, described in DE-A-195 16 784, and 40 g of methyl ethylketone for 1 hour. The resultant kneading compound was subsequentlyadjusted to a solids content of 25% with methyl ethyl ketone in adissolver. This dispersion was subsequently fully dispersed to itsoptimum in a stirred mill (type: Dispermat SL, milling chamber volume125 ml, grinding media zirconium oxide 1-1.25 mm). The progress ofdispersion was monitored by means of an interference contrast technique(EP-B-0 032 710). End fineness was achieved when the surface to betested was free from agglomerates. Into this fully finely dispersemixture there were incorporated, intensively, different amounts of aco-binder, in four portions, using a dissolver. The co-binder used was asulfonate polyurethane (Morthane CA 152, K value=48; M_(w)=20,000; fromMorton International).

Based on parts by weight of carbon black, the following total binderfractions (sum of Disperdur phosphonate and Morthane CA 152) wereformulated in the four batches:

Batch Carbon black: total binder 1 1:2 2 1:3 3 1:4 4 1:5

These differently modified carbon black dispersions were applied in afilm thickness of 0.6±0.1 μm to the cholesteric layer. Apart from the UVcuring, which is not necessary here, the coating process corresponded tothat of stage a).

c) Removing the Layer Assembly from the Substrate Film

The two-layer assembly obtained from stage b) was removed from thesubstrate film by slitting the assembly transversely to the film webdirection, using a razor blade, and then blasting it with compressed airfrom a slot die. The coated film was guided continuously past the slotdie and the assembly removed by blasting was collected in the form offlakes.

d) Grinding the Flakes to a Pigment

10 g of cholesteric flakes, prepared as described under c), were mixedwith 100 g of sodium chloride and milled 6 times for 2 minutes in animpact cutter mill. After grinding, the salt was washed out with waterand the pigment was isolated. The thickness of the pigments obtained wasapproximately 2.7 μm at an average diameter of approximately 25 μm.

e) Mechanical Properties of the Layer Assembly and of the Pigments

For the four batches above, the following parameters were investigated:

(1) Adhesion of the absorption layer to the cholesteric layer

(2) Layer detachment from the substrate

(3) Delamination tendency on pigment grinding

The results are summarized in the table below:

Delamination Adhesion of tendency absorber absorption layer Layerdetachment layer on LC layer Batch to LC layer from the substrate ongrinding 1  7 cN good great 2  8 cN good moderate 3 20 cN good none 4 22cN good none

The adhesive force was determined as follows:

A section of adhesive tape approximately 5 cm in length was affixed to aflat metal plate. The adhesive tape used may be any commerciallycustomary adhesive tape which together with a PET reference film givesan adhesive force under otherwise identical measurement conditions offrom about 7.3 to 7.9 cN, in particular about 7.6 cN. The PET referencefilm used here is a PET film, type E2R, from Teijin, thickness 9 μm,width 6.35 mm, surface roughness according to DIN 4768: Rz=0.86 μm,according to DIN 4786/1: Ra=0.012 μm, in accordance with EP-B-0 032 710(interference contrast technique): 75 to 125 nm. The adhesive layerfaces upward and away from the plate. A sample (length about 10 cm,width 6.35 mm) is cut out from the cured layer assembly still adheringto the substrate film. The assembly to be measured, consisting of thelayer substrate and the two-layer assembly, is applied so that theabsorption layer comes into contact with the adhesive layer. The freeend of the assembly stuck on is then bent so that it forms an angle of160° with the plane of the metal plate. At a constant rate of 0.1 mm persecond, the free end of the strip is then pulled against the stuck-onend of the strip, with a continual increase in the tensile force in thetape until the layer assembly tears. In the subsequent course of themeasurement, the adhesive strength between cholesteric layer andabsorption layer is measured. The tensile stresses in the tape aredetected with a high-resolution sensor and recorded as the peel force incN using a y-T plotter.

We claim:
 1. A platelet-shaped cholesteric multilayer pigment whichcomprises the layer sequence A/B/ and if desired C, where A and Cindependently of one another are at least one partly light-permeableabsorption layer, and B is at least one cholesteric layer; and theadhesive force between the cholesteric layer and the absorption layer ismore than about 8 cN.
 2. A multilayer pigment as claimed in claim 1,where A and/or C comprise at least one organic or inorganic absorptionpigment or an absorption dye, if desired in an organic or inorganicbinder matrix.
 3. A multilayer pigment as claimed in claim 1, where Aand/or C comprise an absorbing polymer layer.
 4. A multilayer pigment asclaimed in claim 1, where A and/or C comprise at least one essentiallyinorganic absorption layer.
 5. A multilayer pigment as claimed in claim1, where the thickness of each individual cholesteric layer of B is from0.5 to 20 μm.
 6. A multilayer pigment as claimed in claim 1, where thethickness of each individual layer of A and/or C is from 1 nm to 5 μm.7. A multilayer pigment as claimed in claim 1, whose diameter is from 3to 500 μm.
 8. A multilayer pigment as claimed in claim 1 having a totallayer thickness of less than 3 μm.
 9. A multilayer pigment as claimed inclaim 1, where B comprises cholesteric mixtures selected from the groupconsisting of a) at least one cholesteric, polymerizable monomer; b) atleast one achiral, nematic, polymerizable monomer and one chiralcompound; c) at least one cholesteric, crosslinkable polymer; d) acholesteric polymer in a polymerizable diluent; and e) at least onecholesteric polymer whose cholesteric phase can be frozen in by rapidcooling to below the glass transition temperature, in the cured state.10. A multilayer pigment as claimed in claim 1, where A and/or Ccomprise a binder matrix comprising at least one cholesteric mixtureselected from the group consisting of a) at least one cholesteric,polymerizable monomer; b) at least one achiral, nematic, polymerizablemonomer and one chiral compound; c) at least one cholesteric,crosslinkable polymer; d) a cholesteric polymer in a polymerizablediluent; and e) at least one cholesteric polymer whose cholesteric phasecan be frozen in by rapid cooling to below the glass transitiontemperature, in the cured state.
 11. A multilayer pigment as claimed inclaim 10, where A, B and if desired C comprise the same cholestericmixtures.
 12. A multilayer pigment as claimed in claim 1 where A and/orC are magnetic.
 13. A multilayer pigment as claimed in claim 1, where Aand/or C and, if desired, B additionally comprise stabilizers to counterthe effects of UV and weathering.
 14. A process for producing amultilayer pigment as claimed in claim 1, which comprises applying thelayers A, B and if desired C atop one another to a substrate,simultaneously or with a time differential, curing the layers, againsimultaneously or with a time differential, removing the layers togetherfrom the substrate and then comminuting them to give multilayerpigments.
 15. A platelet-shaped cholesteric multilayer pigment, whichcomprises at least one cholesteric layer B enveloped by an at leastpartly light-permeable absorption layer A, where A comprises at leastone essentially inorganic absorption layer and where the individualthickness of each individual cholesteric layer of B is from 0.5 to 20μm.
 16. A process for preparing a multilayer pigment as claimed in claim15, which comprises first applying layer B to a substrate, curing it,removing the cured layer from the substrate and comminuting it to formpigments, and then applying the layer A to the pigments.
 17. Acomposition comprising at least one multilayer pigment as claimed inclaim
 1. 18. A coating material comprising at least one multilayerpigment claim
 1. 19. The use of a multilayer pigment as claimed in claim1 in the vehicle and vehicle accessories sector, in the EDP, leisure,sport and games sector, as an optical component such as a polarizer orfilter, in the cosmetics field, in the textile, leather or jewelryfield, in the gift product field, in writing utensils or on spectacleframes, in the construction sector, in the domestic sector or inconnection with a printed product of any kind, for preparing an ink orpaint or for the anticounterfeiting treatment of an article.
 20. The useof a composition as claimed in claim 17 for coating an article ofutility or for painting a vehicle.
 21. The platelet-shaped cholestericmultilayer pigment of claim 15, wherein the thickness of A is from 1 nmto 5 μm.
 22. The platelet-shaped cholesteric multilayer pigment of claim15, wherein B comprises cholesteric mixtures selected from the groupconsisting of a) at least one cholesteric, polymerizable monomer; b) atleast one achiral, nematic, polymerizable monomer and one chiralcompound; c) at least one cholesteric, crosslinkable polymer; d) acholesteric polymer in a polymerizable diluent; and e) at least onecholesteric polymer whose cholesteric phase can be frozen in by rapidcooling to below the glass transition temperature, in the cured state.